The present invention is related to the field of information readout systems, and more specifically to the field of scanning CRT (cathode ray tube) visual display systems which provide for information readout from a memory. The present invention has specific application to scanning CRT graphics display systems.
In known graphics display systems display information is received by a graphics display controller and formatted so as to produce a suitable visual display on a CRT display monitor. More particularly, the graphics display controller formats the information by providing a scanning dot matrix to produce a desired visual image due to the action of at least one scanning CRT gun, with the scanning of the gun across the CRT being reset in response to vertical and horizontal synchronization pulses and the video display being blanked during the reset of the scanning gun. Such scanning CRT displays are very well known and comprise standard CRT display monitors and television sets. To provide a color display, typically three primary color guns are used to simultaneously scan the CRT.
Typically the operation of the graphics display controller is performed in accordance with a received clock signal which determines the frequency of all of the operations performed by the controller. The controller is then programmed to produce the desired results, which may include storing formatted information signals in a refresh memory device and later addressing these stored signals for subsequent readout to the scanning CRT monitor.
In CRT display systems such as those described above, problems arise when it is necessary for the controller to fill the entire refresh memory with video information for subsequent readout. These problems arise because some controllers may not operate at a sufficiently rapid rate to complete the loading of the entire refresh memory within a short loading time period, especially if the time for loading the refresh memory is restricted to the time for blanking of the video display which blanking occurs during the reset of the CRT scanning gun. Typically it is desirable to load the refresh memory only during video blanking and thereby implement a so called "nonflash" mode since loading the refresh memory when the memory is being read out to the video display device may cause undesired flashing of the video image due to the attempted simultaneous read in and read out of video information with respect to the memory. The terms "read in" and "read into" as used herein are synonymous with the term "write" as it is commonly used to indicate the act of storing information in a memory device. This is also referred to herein as a "read mode". The problem of not having enough time for the controller to load the entire refresh memory essentially occurs because the controller is limited in that the maximum ratio of controller read in cycle time for the refresh memory to the controller read out cycle time is fixed, even though the actual times can be set in accordance with the received clock signal. Reading out of video information from the memory typically occurs during a display mode at a predetermined rate typically determined by the clock signal received by the controller, whereas this same clock signal also determines the read in cycle for loading information into the refresh memory and this read in cycle requires additional controller steps besides just addressing the refresh memory which is the only controller step typically required during the display mode.
In prior graphic video display systems such as those described above, four individual signal bits may be used to define each individual dot, such as three individual signal bits each defining the presence of one of three primary color hues which may be associated with the dot, as well as a signal bit defining an intensity characteristic for the dot. This means that these four signal bits which define each dot must be provided simultaneously to the display monitor means which will effectively channel these bits to three different primary color hue scanning electron guns and control the gun intensities. In such systems typically shift registers are utilized to receive predetermined groups of signal bits with each register associated with a dot characteristic such as each of the three primary color hues and an intensity characteristic. The loading of each of these shift registers with data corresponding to defining a number of dots is generally simultaneously performed by utilizing four separate memory devices each connected by an associated data bus to an associated one of these shift registers. Then the output data of the shift registers is sequentially shifted to sequentially define a number of dots. This loading technique for the registers requires a large number of separate data busses, each associated with one of the shift registers and memories, and this greatly increases the expense of such systems. In general terms, this same problem occurs whenever information must be read out of a memory device and provided to a number of different shift registers wherein all of the shift registers are intended to simultaneously provide individual output signal bits which together define a predetermined data word, corresponding to a dot in a scanning CRT system, wherein each of the shift registers has its output data shifted in accordance with a received clock signal so as to sequentially provide a number of such data words.